Loop gain equalizer for RF power amplifier

ABSTRACT

The output power of an RF power amplifier is controlled using a feedback loop including a differential integrator for controlling the amplifier&#39;s bias voltage. The gain of integration in the differential integrator is varied so as to compensate for variations in the derivative of the power amplifier output power versus the bias voltage.

The present invention relates to the field of RF power amplificationand, more particularly, to an RF power amplifier whose output powerlevel is controlled via a bias control loop.

In a radio transmitter it is desirable to be able to control,accurately, the power of the transmitted RF signal so that at any giventime it takes a desired value (for example, a value set by a user). Inparticular, it is desired to avoid fluctuations in output power causedby variations in the frequency of the signal and/or variations intemperature. Generally, the power of the transmitted RF signal iscontrolled to the set value by using a feedback loop to control theoutput power of an RF amplifier used in the transmitter. Moreparticularly, the power of the output RF signal is measured and comparedwith a reference signal so as to produce a difference signal (“errorsignal”). The error signal is used to control, automatically, thebehaviour of the amplifier.

Two main techniques are used for controlling the RF amplifier's outputpower:

-   -   a) controlling the power at the input to the amplifier by        varying the attenuation applied by a voltage-controlled        attenuator provided at the amplifier input—known as an input        power control mode (IPCM), or    -   b) controlling the gain implemented by the amplifier by        controlling the amplifier's bias circuit—known as a bias control        mode (BCM).

An IPCM approach has the advantage that it simplifies the design of thefeedback loop. However, in an IPCM architecture the amplifier applies amaximum gain at all times, which can lead to inefficiency (i.e. in thecase where the output power required is not maximum).

Amplifiers that use a BCM architecture for achieving accurate control ofoutput power are efficient insofar that, if a low output power isrequired, the amplifier gain can be controlled to a low value. However,it can be difficult to achieve a stable loop configuration in view ofthe transfer function between the biasing voltage, Vbias, and the RFoutput power, and in view of the typical dynamic range it is desired toaccommodate. This problem can be better understood from a considerationof the conventional BCM power amplifier architecture illustratedschematically in FIG. 1.

As shown in FIG. 1, in the conventional BCM architecture an RF poweramplifier 1 receives a modulated RF signal, RF_(in), having a powerlevel P_(in), and produces at its output an amplified modulated RFsignal, RF_(out) having a power level P_(out). The power amplifier 1 canhave any desired number of stages of amplification (FIG. 1 illustrates apower amplifier having three stages of amplification). A coupler 2samples the amplifier's output signal RF_(out) and supplies a signalrepresentative of RF_(out) to a power detector 3. The power detector 3produces an output signal V_(det) representative of P_(out) and suppliesthis signal to a differential integrator 4.

In this differential integrator circuit 4, it can be considered thatthere is a notional adder 6 which subtracts the signal V_(det) from areference signal V_(ref) and produces an output signal, V_(error),indicative of the difference. This difference signal, V_(error), is fedto a notional integrator 7. In practice, the differential integratorcircuit 4 is generally implemented as a single device which performs thefunctions of the notional adder 6 and notional integrator 7. The outputfrom the differential integrator 4 is a signal V_(bias) which is used tocontrol the bias circuit 10 of the power amplifier 1. Accordingly, thegain applied by the RF power amplifier 1 is controlled depending on howclose the power level P_(out) of the output signal RF_(out) is to areference value (set by appropriate regulation of V_(ref)).

FIG. 2 is a circuit diagram showing a conventional differentialintegrator 4 a based on an operational amplifier. An integrator of thistype is often used in a BCM architecture such as that of FIG. 1.

The BCM loop approach is used in many radio transmitter applications. Intheory, it should be possible to set the output power level within awide dynamic range using this configuration. However, in order to ensurethat the loop is stable it is necessary to ensure that the loop phasemargin remains at an acceptable value. The phase margin depends on thefrequency location of the open loop unity gain. However, the frequencyat which open loop unity gain is achieved varies according to the loopbiasing.

The open-loop transfer function, H_(ol), is the product of the transferfunction, H_(coup), of the coupler 2, the transfer function, H_(det), ofthe power detector 3, the transfer function, H_(int), of thedifferential integrator circuit 4 and the transfer function, H_(pa), ofthe power amplifier, as defined in equation (1) below:H_(ol)=H_(coup) H_(det) H_(int) H_(pa)  (1)

Assuming that each of the coupler 2, (compensated) power detector 3 anddifferential integrator circuit 4 has a respective constant gain, thenthe overall open-loop gain (K_(ol)) will vary as the power amplifiergain (K_(pa)) varies. In other words:ΔK_(ol)∝ΔK_(pa)  (2)

Now, it is the relationship (transfer function) between the power at theRF output and the amplifier's biasing voltage which determines the gain(K_(pa)) introduced by the power amplifier. FIG. 3A illustrates thisrelationship between P_(out) and V_(bias), whereas FIG. 3B illustratesthe derivative of the relationship, in other words, how∂P_(out)/∂V_(bias) (that is power amplifier gain K_(pa)) varies with theapplied bias voltage, V_(bias).

As it can be understood from FIG. 3, the power amplifier gain varieswidely with the output power. This implies that there is a largevariation in the bandwidth and phase margin of the feedback loop. Thus,it is difficult to find loop parameters which will provide stableoperation of the feedback loop over the whole output power range.

Furthermore, FIG. 3 shows that, at high output powers, the gainintroduced by the power amplifier is actually quite small. Thisdramatically reduces the bandwidth at high power, changing the dynamicsof the feedback loop. In particular, the response of the feedback loopcan become so sluggish that the RF output power level cannot match thetime mask specified by the user (i.e. a required variation of outputpower with time cannot be achieved).

One way of addressing the above-mentioned problems is to increase thebackoff, that is, to make a further reduction in the power actuallyoutputted to the antenna (as compared with the amplifier's rated maximumpossible output power). However, such an approach has the disadvantageof reducing the PAE (Power Added Efficiency).

It is desirable to develop a BCM power amplifier architecture whichavoids the above-described problems associated with the prior art.

The present invention provides a method of controlling the output powerof an RF power amplifier as set forth in the accompanying claims.

The present invention further provides an RF power amplifier as setforth in the accompanying claims.

The present invention yet further provides an RF transmitter as setforth in the accompanying claims.

According to the preferred embodiments of the invention theabove-mentioned problems of a conventional BCM RF power amplifierarchitecture can be addressed by varying the gain of integration of adifferential integrator circuit included in the BCM loop. The advantageof implementing the gain variation at this location within the overallarchitecture is that the desired variation can be achieved in a simplemanner and without conflicting with other design requirements. Moreover,in certain embodiments of the invention the gain variation can beimplemented without increasing the number of physical components in thedifferential integrator circuit.

Features and advantages of the present invention will become clear fromthe following description of preferred embodiments thereof, given by wayof example, illustrated by the accompanying drawings, in which:

FIG. 1 is a block diagram schematically representing the main componentsof a conventional bias-control mode power amplifier architecture;

FIG. 2 is a circuit diagram illustrating the configuration of aconventional operational-amplifier-based differential integrator useablein the architecture of FIG. 1;

FIG. 3 shows graphs indicating how amplifier behaviour varies with theapplied bias voltage in the conventional architecture of FIG. 1, inwhich:

FIG. 3A shows how the amplifier output power varies with bias voltage,and

FIG. 3B shows how amplifier gain varies with the bias voltage;

FIG. 4 is a block diagram schematically representing the main componentsof a bias control mode RF power amplifier architecture according to afirst preferred embodiment of the present invention;

FIG. 5 is a circuit diagram illustrating the configuration of a modifiedoperational-amplifier-based differential integrator useable in thearchitecture of FIG. 4;

FIG. 6 is a graph showing one example of how the input resistance of thedifferential integrator of FIG. 5 can be varied in the first preferredembodiment of the invention;

FIG. 7 is a graph showing how open loop unity gain varies with outputpower of a BCM RF power amplifier architecture according to the firstpreferred embodiment of the invention and a comparative example;

FIG. 8 is a block diagram schematically representing a BCM RF poweramplifier architecture according to the invention in which a firstapproach is used to control the integrator gain; and

FIG. 9 is a block diagram schematically representing a BCM RF poweramplifier architecture according to the invention in which a secondapproach is used to control the integrator gain.

A first preferred embodiment of the invention will now be described withreference to FIGS. 4 to 9.

As shown in FIG. 4, according to the first preferred embodiment of thepresent invention, an RF power amplifier 10 receives a modulated RFsignal, RF_(in), having a power level P_(in), and produces at its outputan amplified modulated RF signal, RF_(out) having a power level P_(out).A coupler 20 samples the output signal RF_(out) and supplies a signalrepresentative of RF_(out) to a power detector 30. The power detector 30produces an output signal V_(det) representative of P_(out) and suppliesthis signal to a bias control circuit 40 which performs the functions ofa differential integrator circuit.

In the bias control circuit 40, a comparator 60 subtracts the signalV_(det) from a reference signal V_(ref) and produces an output signal,V_(error), indicative of the difference. This difference signal,V_(error), is fed to an integrator 70. The output from the integrator 70is a signal V_(bias) which is used to control the bias circuit 80 of thepower amplifier 10.

Once again, as in the conventional architecture of FIG. 1, thecomparator 60 and integrator 70 may be notional devices that areactually implemented using a single differential integrator device, asshall be described in greater detail below. However, in the arrangementaccording to the first preferred embodiment of the invention, the gainof the differential integrator circuit 40 is variable (represented inFIG. 4 using the symbol for a variable amplifier 100). Moreparticularly, the gain applied by the differential integrator circuit 40is varied so as to compensate for variations in the derivative of thepower amplifier output power level versus the biasing voltage. This isachieved by modulating the integrator gain using a coarse inversefunction, that is, using a function approximating to

${K_{x}\left( \frac{\partial P_{out}}{\partial V_{bias}} \right)}^{- 1},$where K_(x) is a constant gain. This approach overcomes theabove-described problems associated with conventional BCM RF poweramplifier architectures while significantly decreasing the amount ofrequired backoff.

The appropriate variation of integrator gain can be implemented in avariety of ways, depending on the differential integrator circuit'sstructure. For example, in the case where a differential integrator 40 ahaving an operational-amplifier based structure of the kind shown inFIG. 2 is used, the required variation in the gain of integration can beachieved as explained below with reference to FIG. 5.

FIG. 5 shows an operational-amplifier-based differential integratorstructure 40 a. The integral gain, K_(i), of such an integrator is givenby the following equation (4):

$\begin{matrix}{K_{i} = \frac{1}{R\; 1 \times C\; 1}} & (3)\end{matrix}$

The integral gain of the differential integrator 40 a is one of thefactors that contribute to the overall open-loop gain K_(ol). If thedifferential integrator's input resistance, R1, is decreased then K_(i)increases and the overall open-loop gain K_(ol) also increases.Decreasing R1, so as to increase open-loop gain K_(ol), at high outputpower levels of the RF power amplifier leads to a compensation for thereduction of loop bandwidth that otherwise would have been observed atthese power levels. Furthermore it simplifies the implementation of astable loop.

In view of the fact that the power amplifier gain is not constant overthe RF output power range, it is advantageous to set the value of thedifferential integrator's integral gain dynamically with a view toachieving the maximum phase margin. By adopting the approach proposed inthe present invention it becomes possible to ensure loop stability whilerequiring a lower degree of backoff. In other words, the RF poweramplifier can operate closer to its nominal maximum output power.

FIG. 6 is a graph showing one example of how the input resistance, R1,of the differential integrator 40 a of FIG. 5 can be varied with outputpower, P_(out), in the first preferred embodiment of the invention. Itwill be seen that the variation in the resistance R1 follows

$\left( \frac{\partial P_{out}}{\partial V_{bias}} \right)^{- 1}$in an approximate fashion. More particularly, in this example theapplied function is smoothed compared to the actual pattern of variationin

$\left( \frac{\partial P_{out}}{\partial V_{bias}} \right)^{- 1},$so as to be piecewise linear.

In fact, in certain applications it may not be necessary for thevariation in gain of integration to follow

$\left( \frac{\partial P_{out}}{\partial V_{bias}} \right)^{- 1}$throughout the whole output power range of the power amplifier. It maybe possible to bring the performance of the power amplifier withinspecification simply by varying the gain of integration according to

$\left( \frac{\partial P_{out}}{\partial V_{bias}} \right)^{- 1}$within a portion of the range of the amplifier's output power (e.g. athigh output power values).

At high output power levels, the architecture according to the firstpreferred embodiment of the invention produces a marked improvement (upto around 10°) in phase margin as compared to the architecture of acomparative example in which integrator gain is fixed.

FIG. 7 illustrates how embodiments of the present invention reduce thevariation in power amplifier open loop unity gain (OLUG) that isgenerally seen in conventional BCM power amplifier architectures. InFIG. 7, first and second traces represent, respectively, the variationin OLUG with output power for a BCM RF power amplifier architectureaccording to the comparative example (having fixed integrator gain) andthe variation in OLUG with output power for a BCM RF power amplifierarchitecture according to the first preferred embodiment of theinvention.

It can be seen from FIG. 7 that the architecture according to the firstpreferred embodiment of the invention is better than the architecture ofthe comparative example at maintaining the open loop unity gain observedat high output power levels close to its level at lower power outputlevels. This increases the dynamic range of the loop and therefore makesit easier to implement a loop that is stable and compliant with the userspecifications.

FIGS. 8 and 9 illustrate two different approaches for exercising dynamiccontrol over the gain applied by the integrator 70 of the bias controlcircuit 40 (whether the integrator 70 is implemented within anoperational-amplifier-based differential integrator structure 40 a as inFIG. 5 or using some other structure). In the examples illustrated inFIGS. 9 and 10, a control module 110 is used to determine what is thepresent value of the function

${K_{x}\left( \frac{\partial P_{out}}{\partial V_{bias}} \right)}^{- 1}.$The control module 110 then controls the gain of the integrator 70 basedon the determined value.

In the example illustrated in FIG. 8, the control module 110 receivesthe signal V_(ref) as an input. Control module 110 treats the currentV_(ref) value as an estimate of the current value of P_(out) and, sincethe curve relating P_(out) and V_(bias) is known, control module 110 candetermine what is the slope

$\left( \frac{\partial P_{out}}{\partial V_{bias}} \right)$of that curve at the location where P_(out) corresponds to V_(ref).(bearing in mind that V_(ref) may be some scaled version of the desiredoutput power, rather than precisely equal to the desired output power).Accordingly, the control module 110 can determine a value for thefunction

${K_{x}\left( \frac{\partial P_{out}}{\partial V_{bias}} \right)}^{- 1}$and controls the gain of the integrator 70 according to this function.

Since the loop is locked, the approach illustrated in FIG. 8 isrelatively simple to implement, even though the control is exercisedbased on what P_(out) should be at a given time, rather than on a directmeasurement of P_(out).

By way of contrast, in the example illustrated in FIG. 9, the controlmodule 110 receives the signal V_(det) as an input V_(det) being a moredirect measurement of the present value of P_(out). Once again, thecontrol module 110 can determine what is the slope

$\left( \frac{\partial P_{out}}{\partial V_{bias}} \right)$of the P_(out)/V_(bias) curve at the location where P_(out) correspondsto the output power represented by the value of V_(det) at this time.Accordingly, the control module 110 can determine a value for thefunction

${K_{x}\left( \frac{\partial P_{out}}{\partial V_{bias}} \right)}^{- 1}$and controls the gain of the integrator 70 according to this function.

This second approach illustrated in FIG. 9 has a double feedback pathand this increases the complexity of the loop design.

Although the invention has been described above with reference topreferred embodiments thereof, the skilled person will readilyunderstand that the present invention is not limited by theparticularities of the above-described embodiments. More particularly,changes and developments may be made to the above-described preferredembodiments without departing from the scope of the present invention asdefined in the accompanying claims.

For example, although the above-described preferred embodiments of theinvention make use of an operational-amplifier-based differentialamplifier structure, the skilled person will readily understand thatother configurations can be used, provided that the functions ofcomparison and integration (with variable gain of integration) areprovided.

Moreover, although the above-described preferred embodiments of theinvention illustrate two particular approaches for controlling thevariation in the gain of integration of an differential integratorcircuit in the BCM loop, the skilled person will readily understand thatthe desired variation in the gain of integration can be controlled inother ways, for example, a look-up table could be used storing the valueof

${K_{x}\left( \frac{\partial P_{out}}{\partial V_{bias}} \right)}^{- 1}$to apply for a given value of V_(ref) or V_(det).

1. A method of controlling the output power of an RF power amplifier,comprising the steps of: providing a bias control feedback loop arrangedto determine the level of the power output by the amplifier and tocontrol the biasing of the amplifier based on the determined powerlevel, said bias control feedback loop including means for comparing theoutput power level with a reference value to produce an error signal andfor integrating the error signal; and dynamically varying the gain ofintegration, applied by the comparing and integrating means, bymodulating the gain of integration using a modulating functioncomprising a function approximately inverse to$\left( \frac{\partial P_{out}}{\partial V_{bias}} \right),$ whereinP_(out) is the output power of the power amplifier and V_(bias) is thebias voltage applied to the power amplifier by the bias control loop. 2.A method of controlling the output power of an RF power amplifier,according to claim 1, wherein said modulating function comprises apiecewise linear function approximately inverse to$\left( \frac{\partial P_{out}}{\partial V_{bias}} \right).$
 3. A methodof controlling the output power of an RF power amplifier, according toclaim 1, wherein said function approximately inverse to$\left( \frac{\partial P_{out}}{\partial V_{bias}} \right)$ constitutesa portion of said modulating function, said portion of the modulatingfunction corresponding to a portion of the range of the amplifier'soutput power.
 4. A method of controlling the output power of an RF poweramplifier, according to claim 1, wherein the providing step comprisesthe step of providing an operational-amplifier-based differentialintegrator and the step of varying the gain of integration comprisesvarying the input resistance to said differential integrator.
 5. Amethod of controlling the output power of an RF power amplifier,according to claim 1, and comprising the step of providing control meansfor determining, based on the value of the reference signal the value ofsaid function approximately inverse to$\left( \frac{\partial P_{out}}{\partial V_{bias}} \right).$
 6. A methodof controlling the output power of an RF power amplifier, according toclaim 1, and comprising the step of providing control means fordetermining, based on a sampled value representative of the amplifieroutput power, the value of said function approximately inverse to$\left( \frac{\partial P_{out}}{\partial V_{bias}} \right).$
 7. An RFpower amplifier comprising: a bias control feedback loop, the biascontrol loop including means for comparing the output power level with areference value to produce an error signal and for integrating the errorsignal; control means adapted, in use, to vary the gain of integrationof the comparing and integrating means, depending on the present levelof output power, by modulating the gain of integration using amodulating function comprising a function approximately inverse to$\left( \frac{\partial P_{out}}{\partial V_{bias}} \right),$ whereinP_(out) is the output power of the power amplifier and V_(bias) is thebias voltage applied to the power amplifier by the bias control loop. 8.An RF power amplifier according to claim 7, wherein the control means isadapted, in use, to apply a modulating function comprising a piecewiselinear function approximately inverse to$\left( \frac{\partial P_{out}}{\partial V_{bias}} \right).$
 9. An RFpower amplifier according to claim 7, wherein the control means isadapted, in use, to apply a modulating function having a portionconstituted by said function approximately inverse to$\left( \frac{\partial P_{out}}{\partial V_{bias}} \right),$ saidportion of the modulating function corresponding to a portion of therange of the amplifier's output power.
 10. An RF power amplifieraccording to claim 7, wherein the comparing and integrating meanscomprises an operational-amplifier-based differential integrator and thecontrol means is adapted to vary the gain of said differentialintegrator by varying the input resistance thereto.
 11. An RF poweramplifier according to claim 7, wherein the control means is adapted todetermine the value of said function approximately inverse to$\left( \frac{\partial P_{out}}{\partial V_{bias}} \right)$ based on thevalue of the reference signal.
 12. An RF power amplifier according toclaim 7, wherein the control means is adapted to determine the value ofsaid function approximately inverse to$\left( \frac{\partial P_{out}}{\partial V_{bias}} \right)$ based on asampled value representative of the amplifier output power.
 13. An RFtransmitter comprising a power amplifier according to claim
 7. 14. Amethod of controlling the output power of an RF power amplifier,comprising the steps of: determining the level of the power output bythe amplifier; controlling the biasing of the amplifier based on thedetermined power level; comparing the output power level with areference value to produce an error signal and for integrating the errorsignal; and dynamically varying the gain of integration by modulatingthe gain of integration using a modulating function comprising afunction approximately inverse to$\left( \frac{\partial P_{out}}{\partial V_{bias}} \right),$ whereinP_(out) is the output power of the power amplifier and V_(bias) is thebias voltage applied to the power amplifier.
 15. A method of controllingthe output power of an RF power amplifier according to claim 14, whereinsaid modulating function comprises a piecewise linear functionapproximately inverse to$\left( \frac{\partial P_{out}}{\partial V_{bias}} \right).$
 16. Amethod of controlling the output power of an RF power amplifier,according to claim 14, wherein said function approximately inverse to$\left( \frac{\partial P_{out}}{\partial V_{bias}} \right)$ constitutesa portion of said modulating function, said portion of the modulatingfunction corresponding to a portion of the range of the amplifier'soutput power.
 17. A method of controlling the output power of an RFpower amplifier, according to claim 14 wherein the step of varying thegain of integration comprises varying the input resistance to anoperational-amplifier-based differential integrator.
 18. A method ofcontrolling the output power of an RF power amplifier, according toclaim 14 and comprising the step of determining, based on the value ofthe reference signal, the value of said function approximately inverseto $\left( \frac{\partial P_{out}}{\partial V_{bias}} \right).$
 19. Amethod of controlling the output power of an RF power amplifier,according to claim 14 and comprising the step of determining, based on asampled value representative of the amplifier output power, the value ofsaid function approximately inverse to$\left( \frac{\partial P_{out}}{\partial V_{bias}} \right).$
 20. Amethod of controlling the output power of an RF power amplifier,according to claim 15, wherein said function approximately inverse to$\left( \frac{\partial P_{out}}{\partial V_{bias}} \right)$ constitutesa portion of said modulating function, said portion of the modulatingfunction corresponding to a portion of the range of the amplifier'soutput power.